The present invention relates to the generation of ozone. More particularly, the invention provides a technique, including a method and an apparatus, for removing oxygen ions (O.sup.+) from an ozone flow, thus increasing the concentration of ozone delivered from an ozone generator to a reaction zone.
Ozone (O.sub.3) is a gaseous allotrope of oxygen derived or formed naturally from diatomic oxygen (O.sub.2) by electric discharge or exposure to ultraviolet light. Ozone is relatively unstable compared with O.sub.2 and is used extensively to purify air and sterilize water. Because ozone is unstable, it is also useful as an oxidizer, typically reacting at a lower temperature (activation energy) than O.sub.2 does in a similar reaction. One example where ozone is used as an oxidizer is in a chemical vapor deposition (CVD) reaction that uses ozone with an organosilane compound, such as tetraethoxysilane (TEOS). The use of ozone allows a layer of silicon dioxide to be deposited on a substrate at a relatively low temperature.
The concentration of ozone may not be critical in some applications, such as water sterilization; however, it is generally desirable to carefully control the ozone concentration in a CVD process. Ozone may be used in a CVD process to fabricate integrated circuits on a semiconductor wafer. For example, ozone and TEOS may be flown into a CVD reaction chamber to form a layer of silicon dioxide. Integrated circuits often have very small surface features, perhaps less than 1 .mu.m in height or width, and fabricating these devices requires careful process controls to ensure high yields. Having a constant, repeatable flow of ozone is important for process control because the ozone concentration is usually matched to a corresponding flow of the reactant gas.
It is also desirable to use a relatively high ozone concentration in some CVD processes. Higher ozone concentrations allow higher flows of corresponding reactant gas which in turn result in a faster deposition rate. The faster deposition rate reduces the time required to deposit a layer of a given thickness, improving efficiency of the CVD process and lowering costs of the resultant wafer. Merely increasing the total output of an ozonator, by increasing the flow through the ozonator, for example, may not solve this problem because the concentration of ozone, rather than total available ozone, may determine the rate of deposition.
Processes requiring ozone obtain the necessary ozone from an ozone generator, or ozonator. The ozonator typically uses electric discharge to produce ozone from O.sub.2. The efficiency of the ozonator depends on many factors, and typically produces O.sup.+ ions, along with the ozone and other molecules and ions. The O.sup.+ ions can react with the ozone to form diatomic oxygen, which not only reduces the efficiency of the ozonator, but can cause the concentration of ozone to decrease, depending on how long the O.sup.+ ions remain in contact with the O.sub.3.
Various methods are used for increasing the output concentration of ozone from an ozonator. One method injects nitrogen (N.sub.2) gas into the input stream of oxygen to the ozonator. It is believed the ozonator produces nitrogen ions, which scavenge the O.sup.+ ions to form a nitrous oxide of the general family NO.sub.x. For example, in one known system, adding 0.1% N.sub.2 to the O.sub.2 at the input to the ozonator increases the ozone concentration at the ozonator output by over 1.5 weight-percent.
FIG. 1 shows a diagram of part of a CVD system that uses ozone from an ozonator with nitrogen injection. A nitrogen source 103 injects N.sub.2 into a flow of O.sub.2 from an oxygen source 102, which flows into an ozonator 101. The ozonator 101 converts some of the O.sub.2 to O.sub.3, which flows through an ozone delivery line 105 into a CVD chamber 106. The flow of gases from the ozone delivery line 105 into the chamber 106 is controlled by a mass flow controller (MFC) 107.
The addition of nitrogen improves ozonator performance, but some of the resultant compounds, for example nitrous oxides, may create problems. For example, the nitrous oxides may combine with ambient moisture to form nitric acid, which can attack components of the ozonator and cause other problems. Therefore, it is desirable to increase and stabilize the concentration of ozone from an ozonator while reducing the concentration of nitrous oxide compounds associated with present schemes.